In most conventional types of heavy-duty excavating machines of the dragline or stripping shovel type, there is usually provided a rotatable frame having a top deck and a bottom plate. A stationary ring gear is mounted to a base of a crawler unit or to a tub which is supported on the ground adjacent to or in a pit of a surface mine. The rotating frame supports at least one drive unit, having a pinion drivingly engageable with the ring gear which operates to swing the rotatable frame. Traditionally, such swing drive units have been mounted on the top deck with the pinion shaft thereof extending through vertical openings in the rotating frame. These existing designs incorporate a long main rotating shaft which connects the gear case mounted on the top deck of the rotating frame to the main rotating pinion located below the bottom plate of the rotating frame. The long main rotating shaft is subject to torsional windup which can cause unbalanced motor load sharing. Severe vibrational problems can also occur in this condition. The overhung main rotating pinion loads also cause the main rotating shaft to deflect in bending sufficiently that the pinion teeth must be heavily crowned to compensate for this misalignment. Additionally, such mounting arrangements have been found not to be entirely satisfactory in that configurations require precise machining of the rotating frame for mounting the swing unit and assuring proper alignment of the pinion shaft with the ring gear.
Large walking dragline machines typically utilize longitudinal bulkheads to support the machine's major digging loads from the gantry front and back leg structures, boom structure, hoist machinery and drag machinery. Transverse bulkheads are utilized to support the machine during the machine's propel operation. Both the longitudinal and transverse bulkheads are blended together at the roller circle bulkhead to support the roller circle loads. Typically this bulkhead configuration consists of an egg crate or waffle type of construction, designed to resist deflection. Such designs present a complex loading and stress pattern in transferring the load from the machinery located on the deck of a dragline to the dragline frame which results in an indeterminate and complex structure for analysis and manufacture.
The precise machining of the rotatable frame and the alignment of the pinion shaft with the ring gear involves a comparatively high manufacturing cost. It is highly desirable to provide a swing drive unit for an excavator which can be effectively and economically installed in the machine in the field and properly aligned with the ring gear of the machine to provide full bearing contact between the teeth of the meshing gears and which provides maximum efficiency in torque transmission.
A need has thus arisen for a rotating frame structure for a dragline which will permit the accommodation of internally mounted swing drive units while simultaneously providing a more simple and efficient frame construction having an easily identified and calculated load paths. A need has further arisen for a rotating frame for mounting swing drive assemblies which provides for uniformity of stiffness around a roller circle and provides uniform distribution of loads in the roller circle.